Stereostatic pointing device

ABSTRACT

A surgical guidance device for the precise positioning of a surgical object comprises a base having three feet which define a first plane and which are adapted for location on markers secured to a patient&#39;s body. The device includes a swivel head which is constrained for movement in a second plane, and a guide for the surgical object. The guide is mounted on the swivel head so that it can swivel with respect to the swivel head in all directions about a pivot point. The device also includes a locking member for locking the swivel head in a selected translational position in the second plane and the guide in a selected angular position with respect to the swivel head. The surgical guidance device forms part of a kit including three markers securable to a patient&#39;s body and on which the feet of the device are respectively locatable.

The invention relates to a surgical guidance device according to thepreamble of claim 1. It also relates to a method of guiding a surgicalobject to a target positioned in a patient.

The surgical guidance device according to the preamble of claim 1 is forexample known from US-A-3,457,922 or US-A-3,073,310.

Both known devices have to be set up whilst attached to the patient'sskull and X-rays are then taken of the patient while the device is soattached. Both devices have skill settings and protractor settings inorder to be able to set up the device. In order to set the known devicesup, the known devices have to be attached to the patient's skull.

According to the invention the surgical guidance device is characterizedby the features of claim 1.

The swivel head may be mounted on a radius arm, the radius arm may bemounted on the base so as to be pivotally displaceable with respect tothe base about a pivot axis normal to the first plane, and the radiusarm may be slidably displaceable with respect to the base or the swivelhead, whereby the radial distance between the swivel head and pivot axiscan be varied.

The guide may comprise a guide tube along which a pin forming thesurgical object or having the surgical object attached thereto, canslide with little clearance, the tube being connected to the swivel headvia a ball-and-socket connection.

The guide tube may have a spherical formation between the ends and onthe outside thereof, and the swivel head may comprise a pair of annularclamping plates between which the spherical formation is held, theclamping plates and the spherical formation forming said ball-and-socketconnection, and locking of the guide in a selected angular position withrespect to the swivel head being effected by clamping the clampingplates together to grip the spherical formation therebetween.

The surgical guidance device may have, in combination therewith as partof a kit, a phantom which comprises a plate having an opening thereinand, spaced around the opening, three locating formations whosepositions relative to one another correspond to the positions of saidfeet relative to one another, and the phantom further comprising atarget block below the opening, the target block being slidable in threemutually orthogonal directions along graduated slides.

Further according to the invention there is provided a method of guidinga surgical object to a target position in a patient, the methodcomprising:

utilising a guidance device having a base with three feet which define afirst plane, a swivel head which is constrained for movement in a secondplane spaced from and parallel to the first plane, and a guide for thesurgical object, the guide being mounted on the swivel head so that itcan swivel with respect to the swivel head in all directions about apivot point, applying at least three markers to the body of the patient,the positions of the markers relative to one another corresponding tothe positions of said feet relative to one another;

determining the co-ordinates of the target position and each of themarkers, in a three coordinate system;

setting up the guidance device by adjusting the translational positionof the swivel head in the second plane and adjusting the angularposition of the guide with respect to the swivel head;

locking the swivel head and the guide in their adjusted positions;

placing the guidance device on the patient so that the feet are inregister with the positions of said markers; and

inserting the surgical object into the patient along the guide.

The invention will now be described in more detail, by way of example,with reference to the accompanying drawings.

In the drawings:

FIG. 1 is a side view of a surgical guidance device in accordance withthe invention;

FIG. 2 is a plan view of the device; FIG. 3 is a section on III--III inFIGS. 1 and 2;

FIG. 4 is a section on IV--IV in FIG. 2, drawn to a larger scale;

FIG. 5 shows the back of a patient's head, to illustrate one of theprocedures in which the guidance device can be used;

FIG. 6 illustrates a setting diagram that is produced by a PC-drivenprinter, for use in setting up the guidance device;

FIG. 7 is a side view, shown partly in section, of an accessory used insetting up the guidance device; and

FIG. 8 is a cut-away pictorial view of a phantom used to verify thesetting of the guidance device.

Referring first to FIGS. 1 to 4, reference numeral 10 generallyindicates a surgical guidance device comprising a base 12. The base 12has three legs 14.1, 14.2, and 14.3, each with a pointed (but not sharp)foot 16.1, 16.2, and 16.3 respectively. The tips of the feet 16.1 to16.3 define a first plane 18.1.

Fast with the base 12 there is an upright post 20. The post 20 is boredto receive a spindle 22 which is rotatable with respect to the base 12about a pivot axis 24, the pivot axis being normal to the plane 18.1.The spindle 22 has a tapered head 26 at the lower end thereof, and athreaded portion 28 at the upper end thereof.

The device 10 further comprises a radius arm 30 which is of squarecross-section and is slidable in a slotted opening 32 in the spindle 22.A wing nut 34 engages with the threaded portion 2B, and between the wingnut and the radius arm 30 there is a spacer ring 36. When the wing nut34 is slightly loose, the radius arm 30 can slide along the slottedopening 32, in the longitudinal direction of the radius arm, and canpivot with respect to the base 12 about the pivot axis 24. When the wingnut 34 is tightened, the radius arm 30 is clamped between the upper faceof the post 20 and the spacer ring 36. This is effective to lock theradius arm in position, locating it against sliding movement in itslongitudinal direction and against pivotal movement about the pivot axis24.

At one end of the radius arm 30 there is a swivel head 38. The swivelhead 38 comprises two annular clamping plates, namely an upper plate 40and a lower plate 42, which can be clamped together by means of a pairof clamping screws 44. The swivel head 36 carries a guide 46 whichconsists of a guide tube 46 having, between the ends thereof, aspherical formation 50 on the outside thereof. The clamping plates 40and 42, in conjunction with the spherical formation 50, form aball-and-socket connection between the swivel head 38 and the guide 46.When the clamping screws 44 are slightly loose, the guide 46 is able topivot in all directions about a pivot point which coincides with thecentre of the spherical formation 50.

The entire device is of stainless steel so that it can readily besterilized.

The manner in which the device 10 is used, for example, to carry out abiopsy procedure on the brain of a patient will now be described, withreference also to FIGS. 5 to 8. To carry out this procedure it isnecessary to bring the tip of a biopsy needle precisely to apredetermined position in the brain of the patient. The guidance device10 has been designed to assist the surgeon in achieving this. Many ofthe steps described below are carried out by assistants outside theoperating theatre, so that they do not take up the surgeon's time.

First, a computed tomography (ACT) scan is taken of the patient's head.Before taking the CT scan, four adhesive marker discs are placed inposition on the patient's head (see FIG. 5). The first one, indicated byreference numeral 60, marks the entry point where the surgeon wishes toenter the patient's cranium. The other three marker discs, indicated byreference numerals 62.1, 62.2, and 62.3, are placed around the markerdisc 60, the spacing between the centres of the marker discs 62.1, 62.2,and 62.3 corresponding to the spacing between the feet 16.1, 16.2, and16.3 respectively. To facilitate the correct positioning of the discs62.1 to 62.3, a template comprising three legs whose configuration isidentical to that of the legs 14.1 to 14.3 may be used. Each of themarker discs 60 and 62.1 to 62.3 has a 2 mm opening at the centrethereof. After the marker discs have been placed in position, a 2 mmsteel ball is placed in each of the 2 mm openings, whereupon the CT scancan commence. The steel balls form radio-opaque markers which arereadily visible on the CT scan images.

The images obtained by the CT scan enable the rectangular co-ordinates(x, y, and z) of any feature shown by the CT scan to be determined. Thex and y coordinates are determined by moving a cursor on the displayscreen when displaying a particular CT slice, to the feature inquestion. The z co-ordinate corresponds to the "table position" of theslice.

A conventional CT scan of the region of interest and consisting of aseries of slices is taken, so as to enable the co-ordinates of the pointin the patient's brain (ie the target) from where the biopsy is to betaken to be determined. The table on which the patient is supported inthe CT scan apparatus is then moved to align one of the steel balls withthe plane of the CT scan apparatus and a scan taken in that particulartable position. This enables the coordinates of the steel ball to bedetermined. The process is repeated for each of the other steel balls,and then once more for the first ball to verify that no significantmovement of the patient's head has taken place between the readings.Once the CT scans have been completed the steel balls can be removed.

The co-ordinates of five positions will then be available, ie those ofthe target, the entry point (corresponding to the centre of the markerdisc 60), and the centres of each of the marker discs 62.1, 62.2, and62.3. With these co-ordinates the guidance device 10 can be set up toguide the biopsy needle precisely to the target.

There are various ways in which the guidance device 10 can be set up.One of these, making use of a suitably programmed PC or lap-top computerlinked to a printer, will now be described.

The co-ordinates of each of the various positions are entered into thecomputer. The computer, making use of the mathematics ofthree-dimensional transformations and co-ordinate geometry, computesfrom this data the co-ordinates, in the plane defined by the centres ofthe marker discs 62.1 to 62.3, of the perpendicular projections on thisplane, of the entry point, the target, and the points at which astraight line passing through the entry point and the target position(the entry-point-to-target line) intersects the second plane 18.2. Thesecomputed positions, as also the positions of the centres of the markerdiscs 62.1 to 62.3 and the point at which the entry-point-to-target lineintersects the plane 18.1, are then plotted by means of the printer on asheet of paper, to form a setting diagram 64 as illustrated in FIG. 6.In the setting diagram the positions of the centres of the marker discs62.1 to 62.3 are indicated by the numbers 1, 2, and 3 respectively. Thepoint at which the entry-point-to-target line intersects the plane 18.2is indicated by the letter P, and the point at which the entry point totarget line intersects the plane 18.1 by the letter D. The settingdiagram also indicates the projected positions of the entry point andthe target (or tumour), as indicated in the drawing.

The setting diagram is placed on a flat supporting surface and theguidance device 10 is placed on the diagram, with the feet 16.1, 16.2,and 16.3 being in register with the points 1, 2, and 3 respectively. Thetriangle defined by the feet 16.1, 16.2, and 16.3 is preferably one thatis not equilateral, so that only one correct position of the guidancedevice 10 on the plot is possible. The wing nut 34 and at least one ofthe clamping screws 44 are slightly loosened. This will enable theswivel head 38 to be moved to any desired position in the plane 18.2,and will also enable the guide 46 to be swivelled in any desired angularposition with respect to the swivel head. The guide 46 is aligned withrespect to the swivel head 38.5 so that the centre line of the guidetube 48 is normal to the planes 18.1 and 18.2. To facilitate this, anaccessory 66 as illustrated in FIG. 7 is provided. The accessory 66comprises a head 68 whose lower end defines a plane 70, and a pin 72which extends at right angles to the plane 70. The distance between thetip of the pin 72 and the plane 70 corresponds to the distance betweenthe upper face of the clamping plate 40 and the plane 18.1 when thecentre line of the guide tube 48 is normal to the plane 18.1. The pin 72is receivable with little clearance in the guide tube 48. The pin 72 isinserted into the guide tube 48 and manoeuvred until the plane 70 liesflat against the upper face of the clamping plate 40. When this is thecase the centre line of the guide tube 48 is normal to the plane 18.1and the tip of the pin 72 will touch or almost touch the setting diagram64. With the wing nut 34 still in a slightly loose condition, the swivelhead 38 is moved in the plane 18.2 until the tip of the pin 72 is inregister with the point P on the setting diagram. The wing nut 34 is nowtightened, thereby locking the swivel head 38 in position in the plane18.2.

The accessory 66 is then removed and a longer pin (not illustrated)inserted into the guide tube 48 until its tip touches the settingdiagram. The pin is now manoeuvred, thereby swivelling the guide 46,until the tip of the pin is in register with the point D on the settingdiagram. The clamping screws 44 are then tightened, thereby locking theguide 46 in position with respect to the swivel head 38.

From the co-ordinates obtained in the CT scan it is also possible todetermine the distance between the upper end of the guide tube 48 andthe target.

This distance can be computed by the computer and printed on the settingdiagram 64.

The guidance device 10 is now ready for use by the surgeon and issterilized before going into the operating theatre.

If desired, the setting of the guidance device 10 can be verified bymaking use of a phantom such as the one designated 80 in FIG. 8. Thephantom 80 comprises a flat plate 82 which has an opening 84 therein andis supported on a bottom plate 86 via legs 88. Spaced around the opening84 there are three small pits 90 in parts of the plate 82 that projectinto the opening 84, for locating the feet 16.1 to 16.3 of the guidancedevice 10. The phantom 80 further comprises a block 92 with a mark 94thereon, the mark 94 representing the target. The block 92 is slidablealong a bar 96. The bar 96 is fixed to a block 98 which is in turnslidable along a bar 100, and the bar 100 is fixed to a block 102 whichis in turn slidable along one of the legs 88. The bars 96 and 100, andthe leg 98 in question are at right angles to one another and are eachmarked with graduations. The blocks 92, 98, and 102 are also eachprovided with a clamping screw (not shown) whereby they can be lockedinto any desired position along the corresponding bar or leg. Thisenables the target block 92 to be adjusted to a position in which theco-ordinates of the target 94 relative to the positions of the pits 90correspond to those of the target in the patient's brain relative to thecentres of the marker discs 62.1 to 62.3.

To verify the setting of the guidance device 10, a long pin (not shown)with a displaceable collar thereon is set up so that the distancebetween the tip of the pin and the collar corresponds to the computeddistance from the upper end of the guide tube 48 to the target. Theguidance device 10 is placed on the phantom 80, with each of the feet16.1 to 16.3 seating in the corresponding pit 90, and the long pin withthe collar thereon is inserted into the guide tube 48 until the collarabuts on the upper end of the guide tube. The tip of the pin should nowbe in register with the target 94. In this manner the surgeon will beable to satisfy himself that the setting of the guidance device iscorrect.

The phantom 80 can be of stainless steel construction, so that it can besterilized for use in the operating theatre.

When the patient is ready for the surgical part of the biopsy procedureto commence, the surgeon will mark the positions of the marker discs62.1 to 62.3 on the patient's skin with an indelible marking fluid. Thiscan be done through the openings left by the removal of the steel ballsreferred to earlier. The marker discs are then removed. If desired,stainless steel marker pins having pits therein for locating the feet ofthe guidance device 10 may be placed on each of the marks and secured inposition by driving them into the bone of the patients skull, so as tofix them with respect to the skull.

The surgeon now enters the patient's cranium at the mark that marks theentry point. Once the patient's cranium has been entered, a biopsyneedle is inserted through the guide tube 48 into the patient's brain.The biopsy needle is provided with an adjustable collar which is clampedin position at a point corresponding to the distance to which it is tobe inserted to reach the target. Thus, when the collar abuts on theupper end of the guide tube 48, the surgeon will know that the tip ofneedle is at the position of the target, whereupon a biopsy can be takenand the biopsy needle withdrawn.

I claim:
 1. A surgical guidance device for the precise positioning of asurgical object, wherein the device comprises:a base having three feetwhich define a first plane, and which are adapted for location onmarkers secured to a patient's body; a swivel head which is constrainedfor movement in a second plane; a guide for the surgical object, theguide being mounted on the swivel head so that it can swivel withrespect to the swivel head in all directions about a pivot point; andlocating means for locking the swivel head in a selected translationalposition in the second plane and the guide in the selected angularposition with respect to the swivel head.
 2. A surgical guidance deviceas claimed in claim 1, wherein the second plane is spaced from andparallel to the first plane and wherein the swivel head is mounted on aradius arm, the radius arm being mounted on the base so as to bepivotally displaceable with respect to the base about a pivot axisnormal to the first plane, and the radius arm being slidablydisplaceable with respect to the base or the swivel head, whereby theradial distance between the swivel head and pivot axis can be varied. 3.A surgical guidance device as claimed in claim 1, wherein the guidecomprises a guide tube along which a pin forming the surgical object orhaving the surgical object attached thereto, can slide with littleclearance, the tube being connected to the swivel head via aball-and-socket connection.
 4. A surgical guidance device as claimed inclaim 3, wherein the guide tube has a spherical formation between theends and on the outside thereof, and wherein the swivel head comprises apair of annular clamping plates between which the spherical formation isheld, the clamping the clamping plates and the spherical formationforming said ball-and-socket connection, and locking of the guide in aselected angular position with respect to the swivel head being effectedby clamping the clamping plates together to grip the spherical formationtherebetween.
 5. A surgical guidance device kit according to claim 1,wherein, in the surgical guidance device, the second plane is spacedfrom and parallel to the first plane, and the feet thereof arenon-sharp.
 6. A surgical guidance device kit, which comprises: asurgical guidance device for the precise positioning of a surgicalobject and comprising a base having three feet which define a firstplane, a swivel head which is constrained for movement in a secondplane, a guide for the surgical object, the guide being mounted on theswivel head so that it can swivel with respect to the swivel head in alldirections about a pivot point, and locking means for locking the swivelhead in a selected translational position in the second plane and theguide in a selected angular position with respect to the swivel head;andthree markers securable to a patient's body and on which the feet ofthe surgical guidance device are respectively locatable.
 7. A surgicalguidance device kit according to claim 6, which includes a phantomcomprising a plate having an opening therein and, spaced around theopening, three locating formations whose positions relative to oneanother correspond to the positions of the feet of the surgical guidancedevice relative to one another, and further comprising a target blockbelow the opening, the target block being slidable in three mutuallyorthogonal directions along graduated slides.
 8. A method of guiding asurgical object to a target position in a patient, which methodcomprises:utilizing a guidance device having a base with three feetwhich define a first plane, a swivel head which is constrained formovement in a second plane spaced from and parallel to the first plane,and a guide for the surgical object, the guide being mounted on theswivel head so that it can swivel with respect to the swivel head in alldirections about a pivot point; applying at least three markers to thebody of the patient, the positions of the markers relative to oneanother corresponding to the positions of said feet relative to oneanother; determining the coordinates of the target position and each ofthe markers, in a three co-ordinate system; setting up the guidancedevice by adjusting the translational position of the swivel head in thesecond plane and adjusting the angular position of the guide withrespect to the swivel head; locking the swivel head and the guide intheir adjusted positions; placing the guidance device on the patient sothat the feet are in register with the positions of said markers; andinserting the surgical object into the patient along the guide.